If such a film is made, it needs to be made in a way that people who haven't seen Doctor Who can understand, just like the old Star Trek films. So, using only David Tannet or Billie Piper, giving them a completely new companion, not mentioning anything that happened in the show, and explaining the origins of the Doctor for the "theater" audience, making it a film they can fully understand.
I don’t think it would go that far, nor need to. At the end of the day, it it’s domestic market there’s more than enough familiarity. (Side note: as with the Bond movies, I laugh when Brit films are declared having a bad domestic box office — by which such discussions usually mean US Box Office.)
I think the old Trek films don’t particularly do that either — there’s zero explanation about the past in TMP or TWOK. TVH had a sort of ‘previously on’ for world markets covering the events of the last two films, but at no point was anything laid out in the films particularly. It was just cultural osmosis.
In terms of Who movies, and why it never really made another jump to screen after the Amicus Dalek films, well — Who fans would want the continuity, and a general audience that might be tempted to see it would expect it to be thee even if it wasn’t these days.
Historically, that meant a totally different continuity (the amicus films, the aborted ‘last of the time lords’ project in the nineties) or simply putting feature length episodes out on a cinematic release. Which has been done well (Day of the Doctor) poorly (the recent finales) and mediocre (also the finales if I am being more kind, but also I think Deep Breath no?)
I think dragging those two back for an attempt to take it to the big screen just plain wouldn’t work — if it’s about international audiences, then Smith and Gillan are bigger in every conceivable way. Even Moffat is bigger internationally than RTD.
All of which is just a long way of saying I don’t think it would work, and would be a misfire to attempt.
Twenty years time as a cinematic reboot? Maybe.
While the show is still nominally in the public consciousness?
Will do more harm than good.